The present invention relates to generators for producing electricity and particularly relates to support plates for the core of the generator having low pressure drop lattice areas to facilitate flow of cooling gas along and through the generator.
In electrical machinery and particularly generators, a pressurized forced gas cooling system, for example, employing air or hydrogen, is frequently used to cool the machinery. The cooling gas must pass through structural elements of the generator supporting the core. Particularly, in generators, section plates, i.e., reinforcing plates, extend generally transversely of the machine axis at axially spaced locations from one another. Openings are formed through the section plates for the transmission of the cooling gas, typically from one end of the generator to its opposite end. Because of cost, shipping and other constraints, the generator/motor supporting frame must be compact and robust. This leaves only very limited space for transmission of the cooling gas throughout the machinery. Cutouts are often provided in the generator support section plates to facilitate transmission of the cooling gas longitudinally along the machinery. Ventilation cutouts, however, reduce the stiffness of the section plates and can result in structural vibration problems. While a tradeoff between affording adequate ventilation through the generator and avoidance of structural and vibration problems is always encountered during frame/ventilation design, the designs often also result in a larger than desired pressure drop of the cooling gas flowing through structural components which do not require cooling, i.e., the non-heat-generating components. These frame pressure drops increase the overall machine ventilation pressure drop, resulting in increased fan head and flow requirements which, in turn, increase windage losses and reduces machine efficiency.
Prior section plate cutouts for flowing the cooling gas have been typically provided as lattice areas formed internal to the section plates. That is, the lattice areas are not bounded by any part other than margins of the openings found in the section plates per se. It will be appreciated that full continuous welds are typically provided between the section plates and the wrapper, i.e., the housing. Typical design practice has been to maintain a substantial separation between the lattice area and the wrapper to provide structural support and facilitate welding between the wrapper and the section plates. In such prior section plates, the lattice area is typically comprised of an array of generally rectilinear and triangularly-shaped openings spaced above the bottom plate of the wrapper by a ligament extending transversely between opposite sides of the generator. Even in those section plates having curved peripheral edges, the lattice area is spaced back from the surrounding wrapper by an arcuate ligament. Analysis has demonstrated that these section plates with lattice area geometries such as described above result in a bending mode of vibration close to twice the running frequency. It will be appreciated that the bottom plate of the wrapper is typically spaced from any floor support by pads along sides and corners of the machinery and significant adverse vibration occurs during operation of the machine. Accordingly, there is a need for an improved structural support accommodating the needs for transmission of cooling flow, as well as structural requirements with reduced vibration.
In accordance with a preferred embodiment of the present invention, there is provided a generator supporting frame structure having a plurality of axially spaced section plates which reduce the ventilation pressure drop through the section plates upon flowing cooling gas, increase the bottom plate bending frequency, reduce the flow coefficient by the elimination of the transverse ligament thereby removing it as an impediment to flow, reduce costs by minimizing the welding required between the section plates and the wrapper and enable reduction of the wall thickness of the bottom plate. To accomplish the foregoing, a plurality of, preferably three, axially extending ribs are provided along the bottom plate of the wrapper. Section plates are also provided at longitudinally spaced positions along the machinery. The section plates have a lattice area formed of a plurality of generally triangularly-shaped openings having transversely alternating apices and bases of the triangular-shaped openings adjacent the bottom plate. With this arrangement, the openings having their bases adjacent the bottom plate also receive axially extending support ribs. The adjacent triangular-shaped openings having their apices adjacent the bottom plate are welded to the bottom plate. The lattice area does not contain any ligament between the truss elements forming a lattice along the bottom plate apart from the apices bounding certain of the triangularly-shaped openings.
By the foregoing construction, gas flow pressure drop through the section plates is substantially reduced by two mechanisms: first, the cross-sectional flow area of the triangularly-shaped cutouts is larger than those openings of the prior art sectional plates and, secondly, the restriction at the interface between the bottom plate and the sectional plates, i.e., the transversely extending ligament, is eliminated, reducing the flow coefficient by a substantial margin. This also facilitates construction of the generator frame by reducing the magnitude of the welding required between the sectional plates and the bottom plate. By lowering the pressure drop, the bottom plate bending mode is moved away from the forcing frequency and the static stress due to internal pressure is lowered.
In a preferred embodiment according to the present invention, there is provided a generator frame comprising a plurality of section plates axially spaced from one another for supporting a core of the generator, a housing about marginal portions of the section plates extending in a direction generally perpendicular to the section plates and including a wall plate, the section plates having a lattice area adjacent the wall plate defined by a plurality of openings through the section plates for flowing gases along and within the generator through the section plates, at least one of said openings through one of the section plates being handled in part by the wall plate.
In a further preferred embodiment according to the present invention, there is provided a generator frame comprising a plurality of section plates axially spaced from one another for supporting a core of the generator, a housing about marginal portions of the sectional plates extending in a direction generally perpendicular to the sectional plates and including a bottom plate, the sectional plates having a plurality of openings through the section plates for flowing gases along and within the generator through the section plates, at least one of the openings through one of the section plates being bounded in part by the bottom plate such that the one opening lies unobstructed in an axial direction by any portion of the one section plate adjacent the bottom plate.